1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method for estimating channel state in a wireless communication system using Fractional Frequency Reuse (FFR).
2. Discussion of the Related Art
FFR is one of techniques that can increase the number of channels per unit area in a cellular system. Typically, a signal gets weaker as it propagates. This means that the same frequency channel can be used at places apart from each other by a certain distance or more. Relying on the principle, subscriber capacity may be increased significantly by simultaneously using the same frequency at a plurality of locations. This efficient frequency use is called frequency reuse.
A cell (or sector) is defined as a geographically distinguished unit area and frequency channel switching between cells to continue on-going communication is called handoff. Frequency reuse is essential to analog cellular mobile communication. A frequency reuse factor is one of parameters that represent frequency efficiency in a cellular system. In a multi-cell environment, the frequency reuse factor is the value of dividing the total number of cells (sectors) using the same frequency simultaneously by the total number of cells.
A first-generation (1G) system (e.g. Advanced Mobile Phone Service (AMPS)) has a frequency reuse factor less than 1. For example, the frequency reuse factor is 1/7 in 7-cell frequency reuse. The frequency reuse factor is higher in a second-generation (2G) system (e.g. Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA)). For instance, Global System for Mobile communications (GSM) being Frequency Division Multiple Access (FDMA) and TDMA in combination boasts a frequency reuse factor of up to ¼ or ⅓. A 2G CDMA or 3rd generation (3G) Wideband CDMA (WCDMA) system may support a frequency reuse factor of 1, thus increasing spectral efficiency and reducing network deployment cost.
The frequency reuse factor of 1 can be achieved when all sectors within a cell and all cells within a network operate on the same frequency channel. Nonetheless, even a system with the frequency reuse factor of 1 may suffer from poor throughput at a cell edge or sector edge due to severe interference between neighbor cells and thus may face service outage. That is, signal reception performance is poor for users at a cell edge because of inter-cell interference.
In Orthogonal Frequency Division Multiple Access (OFDMA), a channel is divided into subchannels and a signal is transmitted on subchannels. Unlike 3G (CDMA2000 or WCDMA), an entire channel is not occupied for signal transmission. Throughput may be increased at the same time for users at a cell center and users at a cell edge by taking advantage of this feature.
To be more specific, a cell center is an area close to a Base Station (BS) that is relatively immune to co-channel interference. Thus users at the cell center may operate on all available subchannels. On the other hand, users at a cell edge are only allowed to operate on a fraction of all available subchannels. This fraction of sub-channels is allocated in such a way that neighbor cells' edges will operate on different sets of subchannels. This is called FFR. The co-channel interference between neighbor cells can be mitigated by orthogonally dividing entire subcarriers into a plurality of Frequency Partitions (FPs) and deploying the FAs such that each cell does not use a certain FA or uses the certain FA at a low power level.
Multiple Input Multiple Output (MIMO) has recently attracted much attention as a broadband wireless mobile communication technology. A MIMO system seeks to increase data communication efficiency by use of a plurality of antennas. Depending on whether the same data or different data are transmitted through antennas, MIMO techniques are classified into spatial multiplexing and spatial diversity.
Spatial multiplexing is characterized in that different data are transmitted simultaneously through a plurality of Transmission (Tx) antennas. Therefore, data can be transmitted at a high rate without increasing a system bandwidth. In spatial diversity, the same data is transmitted through a plurality of Tx antennas, thus achieving transmit diversity. Space time channel coding is a kind of spatial diversity scheme.
Depending on whether a receiver feeds back channel information to a transmitter, MIMO techniques are also categorized into open-loop MIMO and closed-loop MIMO. Open-loop MIMO schemes include Bell Labs Layered Space-Time (BLAST) and Space-Time Trellis Coding (STTC). According to BLAST, the transmitter transmits information in parallel and the receiver detects signals by repeating Zero Forcing (ZF) or Minimum Mean Square Error (MMSE) detection. Thus as much information as the number of Tx antennas can be transmitted. STTC achieves transmit diversity and coding gain by utilizing space. Transmit Antenna Array (TxAA) is a closed-loop MIMO technique.
In a wireless channel environment, channel state changes irregularly in time and frequency, that is, fading is inevitable. Accordingly, a receiver corrects a received signal using channel information in order to recover data transmitted by a transmitter and detect the correct data. The transmitter transmits a signal known to both the transmitter and the receiver to the receiver so that the receiver acquires channel information based on signal distortion created during transmission. The signal is a reference signal or a pilot signal and the process of acquiring channel information is called channel estimation. The reference signal is transmitted with high power, carrying no data. If data is transmitted and received through a plurality of antennas, the receiver should know channel states between the transmission antennas and the reception antennas. Thus, a reference signal is transmitted through each transmission antenna.
Coordinated Multi-Point (CoMP) was proposed to improve the throughput of a user at a cell edge by applying advanced MIMO under a multi-cell environment. The use of CoMP in a wireless communication system may increase the communication performance of an MS at a cell edge. For this purpose, accurate channel estimation needs to be performed based on reference signals received from a plurality of BSs. Multi-cell BSs may provide joint data support to an MS by a CoMP operation. Also, each BS may improve system performance by simultaneously supporting one or more MSs MS1, MS2, . . . , MSK. Further, a BS may implement Space Division Multiple Access (SDMA) based on channel state information between the BS and MSs.
In a CoMP wireless communication system, a serving BS and one or more neighbor BSs, BS1, BS2, . . . , BSM are connected to a scheduler over a backbone network. The scheduler receives feedback channel information representing channel states between the MSs, MS1 to MSK and the BSs BS1, BS2, . . . , BSM, as measured by the BSs. For example, the scheduler may schedule cooperative MIMO information for the serving BS and the one or more cooperating BSs. That is, the scheduler issues a command related to a cooperative MIMO operation directly to each BS.
FIG. 1 conceptually illustrates a CoMP scheme applied to a wireless communication system under a multi-cell environment.
Referring to FIG. 1, there are intra enhanced Node Bs (eNBs) 110 and 120 and an inter eNB 130 in the multi-cell environment. An intra eNB covers a plurality of cells (or sectors) in a Long Term Evolution (LTE) system. Cells covered by an eNB to which a User Equipment (UE) belongs are in an intra eNB relationship with the UE. That is, cells covered by the same eNB that manages a cell in which a UE is located are intra-eNB cells, and cells covered by a different eNB from the eNB that manages the serving cell of the UE are inter-eNB cells.
Cells covered by the same eNB that serves a UE exchange information (e.g. data and Channel State Information (CSI)) through an x2 interface, while cells covered by a different eNB from the serving eNB of the UE exchange inter-cell information via a backhaul 140. As illustrated in FIG. 1, a single-cell MIMO user 150 located in a single cell (or sector) may communicate with one serving eNB in the cell (or sector), and a multi-cell MIMO user 160 located at a cell edge may communicate with a plurality of serving eNBs in a plurality of cells (or sectors).
As described above, eNBs (or cells) perform a CoMP operation for a UE in a multi-cell environment. However, a technique for efficiently estimating interference from neighbor cells to improve the performance of a UE at a cell edge is yet to be specified for an FFR-based CoMP operation under a multi-cell environment.